The scene behind a sharply focused close-up of a subject will normally be unsharp, since this is the experience afforded by typical photographic lenses and the lens of the human eye. However, when the subject is at thirty feet or more, the background is expected to be in reasonably sharp focus.
Improvements in blue screen composite photography for removal of the blue backing, and improved techniques for detecting and scaling camera movements, has made practical the use of a virtual studio for live television broadcasting.
The virtual studio replaces backgrounds made of lumber, paint and wallpaper, with background scenes created by an artist or photographer. Being an image, the background does not change focus as the foreground camera focus follows the subject as it approaches or recedes from the camera. This lack of focus-change of the background, as the camera changes focus, has been one of the flaws in the virtual studio concept. This invention automatically defocuses the background image as the subject approaches the camera, thus simulating what would have occurred if the background had been real.
An actor, when distant from the camera, occupies only a small area on the image frame. When the actor is close to the camera (as for a close-up), he fills a large part of the image frame. The ratio of subject area to image frame area is one of the criteria for estimating the extent of background defocus that should occur at various subject to camera distances.
The area of the subject is provided by the compositing matte signal. Adjustable ratio limits are set to initiate and terminate the defocus being generated. The defocus and refocus rate is controlled when an actor enters or leaves the frame when near the camera.